This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Endomembranes of eukaryotic cells are dynamic structures that are in continuous communication through the activity of specialized cellular machineries such as the coat protein complex II (COPII), which mediates cargo export from the endoplasmic reticulum (ER). COPII is comprised of the Sar1 GTPase, Sec23/Sec24 (Sec23/24), where Sec23 is a Sar1-specific GTPase activating protein and Sec24 functions in cargo selection, and Sec13/Sec31 (Sec13/31), which has a structural role. While recent results showed that Sec23/24 and Sec13/31 can self-assemble to form COPII cage-like particles, we have now shown that Sec13/31 can self-assemble to form minimal cages in the absence of Sec23/24.